(1) Field of the Invention
The present invention relates to an automatic screen printing process in which the repeat length of an endless belt is preset as a pulse number and a roller for driving the endless belt is intermittently driven based on this preset pulse number. More particularly, the present invention relates to a method and apparatus for effectively correcting feed errors in the endless belt in this automatic screen printing process.
(2) Description of the Prior Art
In an automatic screen printing machine, an endless belt for intermittently feeding a cloth to be printed, in the state applied thereon, is ordinarily prepared by piling several numbers of fiber layers and rubber layers alternately and compressing the piled layers to form an integrated structure. Accordingly, the endless belt is not rigid, and therefore, it is not uniform in the dimensional precision and mechanical properties.
Accordingly, however high the operation precision of an intermittent feed mechanism may be in the automatic printing machine, errors are inevitably caused in the intermittent feeding owing to the following inherent properties of individual endless belts:
(1) Uneven thickness (thickness difference more than .+-.0.1 mm per 3.0 mm thickness) PA1 (2) Uneven elongation (elongation difference more than .+-.0.1% per 0.7% elongation)
Actually, the uneven elongation has worst influences on precise intermittent feeding of an endless belt as well as the uneven thickness.
More specifically, as shown in FIG. 3, an endless belt 3 is intermittently driven by a driving roller 1 arranged in the rear portion of the printing machine, and in this mechanism, it has been confirmed that feed errors are caused by changes of an imaginary driving diameter of the endless belt wound on the periphery of the driving roller (D in FIG. 3; called "pitch circle diameter" or "center face"), that is, changes of the height h from the outer diameter of the driving roller and changes of the quantity of elongation of the front part of the endless belt that is wound on the driving roller.
As means for correcting such feed errors for maintaining a required accuracy in feeding, there has been adopted a method in which, of the entire circumference of the endless belt, a part of a large elongation is backcovered with a cloth having a thickness corresponding to the elongation of said part to ensure a uniform elongation as much as possible. However, this method is defective and disadvantageous in that the operation of back-covering the endless belt is very difficult and troublesome and lives of applied cloths are very short because they are readily peeled or broken. Furthermore, an effect of maintaining a required accuracy in the feeding operation is not complete. Therefore, this method is still insufficient.
As another means for eliminating feed errors, there has been proposed and manufactured a printing machine in which both the selvage ends of an endless belt are clamped as shown in FIG. 4 and a device for clamping the selvage ends is intermittently fed with high precision by a hydraulic cylinder or the like. The printing machine of this type is defective and disadvantageous in that the structures of the clamping device and feed mechanism are much complicated and when the width of the endless belt is large, even if both the selvage ends are precisely fed, the central portion of the belt is delayed by a distance S behind the selvages by the resistance owing to the frictional contact with a printing table and is fed in an arcuate shape as shown in FIG. 4. Accordingly, a required precision cannot be maintained in the central portion of the endless belt. Moreover, uneven forces are imposed on the endless belt to cause deformation and shorten the life of the endless belt.
The applicant has already proposed an automatic screen printing machine of the roller intermittent drive type in which the repeat length of an endless belt is preset as the pulse number, the actual feed length of the endless belt is detected as a pulse number, subtraction of the actually detected number of pulses from the preset pulse number is performed and control of driving is carried out so that an electric motor for driving the roller is stopped at the preset pulse number (see Japanese Patent Application Laid-Open Specification No. 34483/79). In the automatic printing machine of this type, there can be attained an advantage that the repeat length can be preset in a broad range in a non-staged manner very simply by operating switch means, and the automatic printing machine of this type is excellent over the conventional printing machine with respect to the feed precision. However, it is technically difficult to detect the actual feed length of the endless belt precisely as the number of pulses, and this automatic printing machine is still insufficient in the effect of completely eliminating feed errors caused by uneven thickness or uneven elongation of the endless belt per se.
For example, in the case where the actual feed length of the endless belt is detected as the number of pulses by a measuring roll falling in contact with the endless belt and a pulse generator coupled with the measuring roll to generate pulses in a number corresponding to the displacement of the measuring roll, precise detection of the feed length is difficult because of deformation of the endless belt per se or slips caused between the endless belt and measuring roll. When the pulse generator is directly connected to an endless belt-driving roller free of such slips or a driving direct current electric motor, it is also difficult to detect the actual feed length of the endless belt precisely because of changes of the above-mentioned pitch circle diameter or changes of the elongation quantity in the endless belt.